I assert they can, because they have, and still do. It may be that a takings challenge to drug prohibition has never been made. No compensation has ever been paid by any government for loss of property by enforcement of a drug law. I doubt compensation has ever been paid for enforcement of any law deeming a class of property contraband.

SCOTUS said in Whipple v. Martinson "There can be no question of the authority of the State in the exercise of its police power to regulate the administration, sale, prescription, and use of dangerous and habit-forming drugs..." Sounds like a statement of near-unbounded authority to ban possession of previously lawful property, which is what anti-drug acts are and were.

Why would you think guns can get better treatment?

Because people legally own those guns right now. I seriously doubt anyone owns drugs from before they were made illegal.